At present, lithium-ion batteries have been widely applied in various kinds of portable devices due to their high output voltage, high energy density and long cycling life. Meanwhile, lithium-ion batteries have been appreciated in many fields such as military affairs, spaceflight, electric vehicle, etc.
At present, a negative electrode in a conventional lithium-ion battery mainly comprises graphite, however, a lithium intercalation potential of a graphite negative electrode is lower than a reduction potential of electrolyte solvents such as PC, EC and DEC, therefore the solvent may be reduced to form a solid electrolyte film (SEI) on the surface of negative electrode. Not only electrons but also lithium ions take parts in the reaction, to form products such as Li2CO3, LiF and alkyl lithium carbonate. The graphite negative electrode originally has no lithium ions, and lithium source in the negative electrode is from positive electrode material. However, the formation of SEI film may lead to 7%-10% loss of active lithium ions, especially lithium ions in oxides and alloy nanometer materials of the negative electrode material, and more lithium ions may be lost due to a combination of oxide ions and lithium ions, and also due to that a specific surface of the nanometer material is much larger than that of graphite. A loss of lithium may cause lithium ions extractable from the negative electrode to be extracted from the negative electrode completely at a later stage during a subsequent charging-discharging cycle. As the positive electrode still has available lithium insertion vacancies, an electric potential of the positive electrode may keep stable or fall slowly, while an electric potential of the negative electrode may rise quickly because lithium ions thereof has been extracted completely. When the electric potential of the negative electrode is above 1V, a part of the positive electrode SEI film may be damaged. The current collector copper at the negative electrode may be dissolved if the electric potential of the negative electrode rises. The above copper dissolution and damages on SEI film may cause bad influences on the battery performance.
In the related art, in order to prevent damages on the negative SEI film and a dissolution of the copper current collector, the negative electrode must be rich in lithium ions. For example, Chinese patent application publication No. 1290209C discloses a method of lithium supplement. The method comprises: mixing metal lithium, negative electrode material and a non-aqueous liquid to from a slurry, coating the slurry onto a current collector, and then drying, rolling the current collector and injecting an electrolyte. The above method may improve a power density of lithium-ion battery. However, as metal lithium has a high reactivity and tends to react with oxygen and water in the air, the method not only requires a non-aqueous organic solvent that may not react with lithium ions, but also have a restrict requirement on the controlling of water in the preparing process, thereby increasing the process difficulty. The above matters also exist in other patent applications, such as JP1996027910, JP2005038720, CN200610089725 and CN201210351225. In addition, lithium-ion battery has a high requirement on purity of all materials, while the above lithium supplement method may bring side reactions, the impurities of which may have bad influences on the battery performance. A stabilized lithium metal powder (SLMP) produced by American company FMC may be used for lithium supplement, the SLMP may be obtained by modifying a surface of a lithium powder. The SLMP may stay stably in dried air just for several hours, which may limit operation time for the preparation of a battery, including slurry preparation, coating, drying, cutting, rolling and winding. In addition, the SLMP has a strict requirement on environment humidity and oxygen content. If the SLMP is used for lithium supplement, lithium powder may float in the air if a dry process is taken, which may bring a relative higher security risk; and a problem of using non-aqueous organic solvent and controlling of water may exist if a wet process is taken.